Objective - To describe the surgical treatment of a dorsal diaphragmatic hernia in an adult horse using thoracic rib resection aided by thoracoscopy and a flank incision. Study Design - Case report. Animal Population - One client-owned horse. Results - A six-year-old Dutch warmblood gelding was admitted for colic-associated colonic incarceration in a diaphragmatic hernia. Seven weeks after the initial colic surgery, the gelding underwent surgery to repair the defect. Thoracoscopy and a flank incision were used to identify the location of the hernia and the subsequent site of rib resection. The stomach was adhered to the edges of the defect, leaving only a small residual hole. Resection of the 14th rib provided excellent exposure of the defect and closure of the remaining hernia. Conclusion and Clinical Relevance - Thoracic rib resection can provide access to diaphragmatic hernias in adult horses. Thoracoscopy or a flank incision, or both, may aid in determining which rib is best resected.